PART 1. CHAPTER I. 



23 



Contemporaneous origin of Four Classes of Rocks. 



great divisions of rocks. The strata from a to z represent as 

 many distinct aqueous formations, which have originated at dif- 

 ferent periods, and are each distinguished by their pecuHar fos- 

 sils. The mass i; is of volcanic origin, and was formed at one 

 of those periods, namely, when the strata g were deposited. The 

 strata m m are ancient metamorphic formations, and the rocks 

 1, 2, are plutonic, also ancient, but of different dates. 



Now it will be shown in the course of this volume, that por- 

 tions of each of these four distinct classes of rocks have origin- - 

 ated at many successive periods. It is not true, as was formerly 

 supposed, that all granite, together with the crystalline or meta- 

 morphic strata, were first formed, and therefore entitled to be 

 called " primitive," and that the aqueous and volcanic rocks were 

 afterwards superimposed, and should, therefore, rank as second- 

 ary in the order of time. This idea was adopted in the infancy 

 of the science, when all formations, whether stratified or unstra- 

 tified, earthy or crystalline, with or without fossils, were alike 

 regarded as of aqueous origin. At that period it was naturally 

 argued, that the foundation must be older than the superstructure. 

 Granite, as being the lowest rock, must have been first " precipi- 

 tated from the waters of the primeval ocean which originally 

 invested the globe," then the crystalline, and finally the fossilife- 

 rous strata, together with other associated rocks, were deposited. 



But when the doctrine of the igneous origin of granite was 

 generally adopted, the terms primitive and primary, as embra- 

 cing thfe plutonic and metamorphic rocks, should at once have 

 teen banished from the nomenclature of geology. For after it 

 had been first proved that granite had originated at many differ- 

 ent epochs, some antecedent, others subsequent to the origin of 

 many fossiliferous strata, it was also demonstrated that strata 

 which had once contained fossils, had become metamorphic at ' 

 different periods ; in other words, some of the rocks termed pri- 

 mary were newer than others which were called secondary. A 

 question, therefore, has arisen, whether the lower crystalline por- 

 tions of the earth's crust, partially modified as they have been, 

 and renewed from time to time, are newer or older, regarded as 

 a whole, than the sedimentary and volcanic formations. Have 

 the operations of decay and repair been most active above or 

 below 1 The same question might be asked with respect to the 

 relative antiquity of the foundations and the buildings in certain 

 ancient cities, such as Venice or Amsterdam, which are supported 

 on wooden piles — whether in the course of ages have the wooden 

 props, or the buildings of brick, stone, and marble which they 

 support, proved the most durable ? Which have been renewed 

 most frequently ? for the piles, when rotten, can be removed one 



